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Earth
Earth''' is a medium sized terrestrial class planet and the third world orbiting the star Sol. It is orbited by a large, inhospitable moon known as Luna. Earth is perhaps best known for being the homeworld of humanity. '''Overview The Earth is believed to have been formed 4.5 billions of years ago, along with the rest of Sol's planetary system. During the course of its formation, it is believed that Earth collided with another planetary object, about the size of Mars, which resulted in the destruction of this object and the removal of a large chunk of the newborn Earth. This former portion of the planet stayed in orbit and eventually formed its moon, Luna. After some half a billion years, Earth's surface cooled off, allowing for the formation of large oceans of liquid water. Some of the water had come to Earth via impacts of ice comets. The minerals dissolved in the oceans account for the sea salt. The first traces of life appeared soon afterwards. The planet's magnetic field is believed to have established itself about 3.5 billion years ago, helping to protect the ancient planet from solar radiation and other threats. Exactly how life appeared on the planet is still a mystery. Some theories involve it having naturally evolved from complex chemical compounds in a primordial soup, this happening either near the ocean surface or near the hydrothermal sea vents in the deep sea, or still possibly in the atmosphere. Other theories state that life arrived on Earth from somewhere else, either naturally, as in the Panspermia Hypothesis; or with help from a sapient alien species, as in the Exogenesis Hypothesis. Among the many sorts of unicellular creatures which evolved on Earth, some were phototrophs, using sunlight to perform photosynthesis, which involves the consumption of carbon dioxide and the ejection of free oxygen in the atmosphere. This oxygen was fatal for most of the lifeforms then, but some evolved to consume the new gas, using it for respiration. These oxygen-breathing microbes spread through the planet and gave origin to most of the extant earthling lifeforms. Wildlife All life on Earth is carbon-based and has a very complex biochemistry involving several chemical elements, notably hydrogen, oxygen, silicon and nitrogen. Main groups of organisms on the planet include viruses, bacteria, archaeobacteria and eukaryotes. The last group is believed to have been formed by means of symbiogenesis between bacteria and archaea. Within the eukaryotes several complex organism groups are found, including the phototrophic algae and land plants, the decomposing fungi and the predatory animals. Single celled eukaryotes are also notable for their variety of forms and lifestyles, but are usually clumped together in the artificial group known as protozoa, nevertheless. Properties Earth is the largest terrestrial planet of the Solar System and has the greatest density of any planet in the system as well. It is a geologically active planet, with an iron-nickel core surrounded by a mantle of liquid magma and covered by a cold rocky crust. 70% of the surface of Earth is covered by oceans of salt water. There are currently six main continents: North America, South America, Africa, Oceania, Antarctica and Eurasia. Some sources list Eurasia as two different continents: Europe and Asia; and some have the Americas taken together as one sole continent. Also, the lower portion of North America is sometimes called the Central America and treated as a continent on its own. However, all these views are mostly based on Human politics and sociology. Due to the constant movement of the continental plates, the continents' arrangement was different in each past period of Earth. Earth also has two polar caps, one on the north, creating the frozen area known as the Arctic Ocean; and one on the south, blanketing the continent of Antarctica. The atmosphere is composed of 78% nitrogen and 21% oxygen, which is produced by phototrophic lifeforms such as plants and phytoplankton. Several other elements are also present.